Neuropathy is a pathology that affects the peripheral nervous system with the exception of cranial nerve I and II, i.e. the olfactory nerve and the optic nerve respectively. Neuropathy may be localized in one nerve (mononeuropathy) or in several nerves (polyneuropathy).
There are various types of degeneration that affect the nerve fibre:    (i) Wallerian degeneration as a result of interruption of the axon or of the nerve with maintenance of connective tissue: after about 48 hours there is swelling of the axon with myelin destruction and phagocytosis of globular lipid formations; after some days, budding is observed, attributable to the attempt at regeneration with recovery of about 1 mm daily.    (ii) Axonal degeneration as a result of metabolic changes: affects the distal parts of the fibre initially with slow progression in the proximal direction.    (iii) Segmental demyelination through primary damage to the Schwann cells through metabolic changes, inflammatory process, toxic damage: there is a gradual slowing of the impulse, as far as block when demyelination affects a length of fibre greater than 3 internodes, there is axonal distress and an attempt at remyelination on the part of new proliferating cells, but the succession of processes of demyelination and remyelination causes thickening of the fibre by interposition of fibroblasts and collagen, with formation of fibres with “onion bulb” palpable on the skin (Dejerine-Sottas hypertrophic, amyloidotic and chronic inflammatory demyelinating polyneuropathies).
Based on the aetiology, neuropathies may be classified as follows:                demyelinating inflammatory neuropathies (for example Guillain-Barré syndrome);        neuropathies due to infective agents (for example HIV, Epstein-Barr, leprosy, sarcoidosis);        metabolic neuropathies (diabetes, porphyrias, hypothyroidism);        toxic-nutritional neuropathies (alcohol, hypovitaminosis, drugs, toxic metals);        paraneoplastic neuropathies;        paraproteinaemic neuropathies (amyloidotic, gammopathies, cryoglobulinaemias);        neuropathies in the course of connective-tissue diseases;        hereditary neuropathies (for example Charcot-Marie-Tooth syndrome).        
The peripheral neuropathies are thus a heterogeneous group of diseases of the peripheral nerves. There are multiple causes and, as stated above, they are represented by hereditary and metabolic factors, oxidative stress on the nerve tissue, trauma, infections and inflammations. Most of the peripheral neuropathies are characterized by symptoms such as pain, muscular weakness and sensory loss, which are manifested in general motor disability of the patient.
Neuropathic pain is pathological in that it does not represent a useful and protective function for the organism; in fact, it is characterized by a process of amplification of the nociceptive messages that may be manifested both in the peripheral and in the central nervous system.
In contrast to somatic pain, which arises from particular nerve endings (the pain sensors located in the dermis) and is perceived through tissue damage, neuropathic pain arises directly from a dysfunction of the nerves and does not imply damage in progress.
Neuropathic pain constitutes a common symptom in peripheral neuropathies depending on the length of the nerve, and often represents their initial symptom. Therefore peripheral neuropathies can be defined as a pathological process that mainly affects the myelin fibres of small diameter or the unmyelinated fibres.
Neuropathic pain is associated with numerous types of sensory signs and symptoms that may be manifested alone or together with other specific manifestations in patients with neuropathic pain.
Among the various aetiopathogenic mechanisms hypothesized and demonstrated on the basis of the signs and symptoms of the mechanical peripheral neuropathies, we may mention alteration of the sodium channels (diabetic neuropathies), hyperexcitability of the neurons, sudden changes in spinal connectivity, strong oxidative stress on the nerve tissue. The mediators of inflammatory processes also seem to have a specific role in the onset of degenerative and inflammatory neuropathies.
To summarize, the causes of painful symptomatology of the upper and lower limbs are numerous and of varied origin, and may be of orthopaedic interest (myalgias, compressive or traumatic neuropathies). In fact, a high percentage of these painful syndromes of the limbs are to be attributed to irritative or compressive radiculopathies, with cervical relevance in the case of the upper limbs, and lumbosacral with respect to the lower limbs, the commonest cause of which is a herniated disc.